For wireless communications devices, information terminals and other electronic devices employing batteries as their power source for which the convenience of portability is required, there is a demand for a long time use after charging when the battery is a secondary battery or after battery exchange when the battery is a primary battery, in addition to compactness and lightweightness.
The configuration shown in FIG. 19 commonly is used for such a battery-driven electronic device. As shown in FIG. 19, a conventional battery-driven electronic device uses as its input source a battery typified by a conventional lithium ion battery, more specifically, a battery 401 employing lithium cobaltate as the positive electrode, graphite as the negative electrode, and a nonaqueous solvent containing lithium supporting electrolyte as the electrolyte and having an average battery voltage of 3.7 V at room temperature as the discharge characteristics, and supplies a predetermined voltage Eo to a load 406 via a step-down converter constituted by switching means 402, rectification switching means 403, an inductor 404, and an output capacitor 405.
Hereinafter, the terms will be defined. An “average battery voltage” is defined as a voltage obtained by dividing the time integration of the battery voltage when discharge is performed at a rated current from the rated charge state to the rated discharge capacity by the discharge time. A “rated charge end voltage” is defined as a battery voltage at the end of a charge operation necessary to obtain the rated discharge capacity. A “rated discharge end voltage” is defined as a battery voltage when the rated discharge capacity is obtained. A “rated discharge last stage voltage” is defined as a battery voltage when a large change occurs in the slope of the discharge voltage characteristics during discharging at the rated current.
First, the operation of the step-down converter will be described. When the voltage of the battery 401 is taken as Ei and the switching means 402 is turned on, a difference (Ei−Eo) between the output voltage of the battery (hereinafter, referred to as a “battery voltage”) Ei and a supply voltage Eo is applied to the inductor 404. In this case, current flows from the battery 401 to the output capacitor 405 via the switching means 402 and the inductor 404. The current in the inductor 404 increases, so that magnetic energy is accumulated in the inductor 404. This period is taken as Ton.
Then, when the switching means 402 is turned off, the voltage of the inductor 404 is inverted, and the rectification switching means 403 is turned on, and thus the supply voltage Eo is applied to the inductor 404. In this case, the current in the inductor 404 flows through the rectification switching means 403 to the output capacitor 405. This current is decreased, and the magnetic energy accumulated in the inductor 404 is released. This period is taken as Toff.
When the accumulation and the release of the magnetic energy is in equilibrium through such an on/off operation of the switching means 402, the increase and the decrease in the current flowing in the inductor 404 is in equilibrium. When the inductance of the inductor 404 is taken as L, the following relationship is satisfied:(Ei−Eo)·Ton/L=Eo·Toff/L
When the switching cycle T=Ton+Toff, the following relationship is satisfied between the input and the output of the step-down converter:Eo/Ei=Ton/T
That is to say, in the step-down converter, the supply voltage Eo to the load 406 can be adjusted under the limitation of Ei>Eo with respect to variations of the battery voltage Ei by the on/off control of the switching means 402.
On the other hand, when the voltage Eo that should be supplied to the load 406 is higher than the battery voltage Ei, a step-up converter is used instead of the step-down converter. The configuration using a step-up converter is disclosed, for example, in JP 4-315320 A.
The conventional lithium ion battery as the battery 401 is characterized in that the discharge characteristics are flat, that is, a reduction in a battery voltage associated with discharge is small (the battery voltage change ratio is small). Thus, the battery voltage can be close to the voltage that is required to be supplied to the load 406, and the loss in the step-down converter can be decreased. In addition, this relationship does not change with discharging, so that the energy of the battery can be used efficiently.
In the configuration of the conventional battery-driven electronic device, when a step-down converter is used, the battery voltage Ei should be higher than the voltage Eo that is required to be supplied to the load. On the other hand, it is better that the battery voltage Ei is close to the supply voltage Eo in order to operate the step-down converter efficiently. When a step-up converter is used, the battery voltage Ei should be lower than the voltage Eo that is required to be supplied to the load. In this case as well as in the case of the step-down converter, it is better that the battery voltage Ei is close to the supply voltage Eo in order to operate the step-up converter efficiently.
Therefore, the characteristics required for batteries are that the battery voltage Ei is slightly higher or slightly lower than the supply voltage Eo and that the discharge characteristics are flat. For batteries, research has been carried out for achieving large capacity while satisfying these characteristics. The battery use time, which is a period until the battery voltage that decreases with discharging reaches the lower limit of the voltage at which the battery can be used, has been prolonged by achieving a large capacity with the weight and volume unchanged.
The flatter the discharge characteristics are, the more significant a decrease of the battery voltage tends to be when approaching the last stage of discharging. That is to say, the energy that is left at this point is small. The conventional lithium ion battery employing lithium cobaltate as the positive electrode, graphite as the negative electrode, and a nonaqueous solvent containing lithium supporting electrolyte as the electrolyte and having an average battery voltage of 3.7 V at room temperature has the most excellent discharge characteristics at present.
However, although battery-driven electronic devices have become more compact and high performance and there is a demand for improvement in compactness and large capacity of batteries, that is, improvement in energy density, the theoretical limit is almost reached in terms of large capacity for the above-described conventional lithium ion batteries. Then, a lithium ion battery employing a new material that allows improvement of the energy density of batteries is necessary. However, the batteries made of a material that can realize a high energy density have problems in that the average battery voltage is lower and the battery voltage change ratio is larger than those of the conventional lithium ion batteries.
When the lithium ion battery employing such a new material is incorporated into equipment in which the conventional lithium ion battery is used, and the supply voltage required by the load in the electronic device is between the rated charge end voltage and the rated discharge end voltage of the battery, then the following problem arises. The battery voltage is decreased with discharging, and thus the time at which the battery voltage becomes lower than the supply voltage required by the load is reached earlier than the conventional lithium ion battery, so that the advantage of the high energy density resulted from the new material cannot be utilized. Consequently, the use time of the equipment is shorter than that of the conventional lithium ion battery.
Furthermore, it is considered that two cells, each of which is a lithium ion battery made of a new material having a low average battery voltage and a large battery voltage change ratio, are used to increase the battery voltage and are combined with a step-down converter. However, the voltage difference between the input and the output becomes large, so that it is difficult to realize high efficiency of the step-down converter, and the input voltage is increased. Therefore, it is necessary to increase the withstand voltage of capacitors or semiconductors, and therefore the efficiency is further decreased and the components become larger so that compactness cannot be achieved.